As a technique for measuring a stereoscopic shape of an object being transferred, there is a light-sectioning method.
Specifically, the light-sectioning method is performed in a manner shown below.
First, a sheet-like or linear light beam is irradiated in a width direction of an object being transferred (a direction orthogonal to a transfer direction of the object), from a light source which is disposed above the object so as to be inclined at a predetermined angle relative to the transfer direction.
Since the light source is inclined relative to the transfer direction, the position of an image formed by the light beam from the light source changes in accordance with the projection and recess of a surface of the object. A light-section line which is an image formed by reflection at the surface of the object is captured by a camera or other imaging devices mounted above and vertically to the object.
The shape of the object on the light-section line is detected based on a positional relationship between the light source and the imaging device and the light-section line on the image.
By continuously detecting the shape of the object on the light-section line in a length direction of the object or continuously detecting the shape of the object moving in the transfer direction, the overall three-dimensional shape of the object can be measured.
In the light-sectioning method, it is important to eliminate the influence of disturbance or noise from an image where a light-section line is captured, to extract the light-section line accurately and sharply.
To attain this objective, various propositions have been made.
For example, in JP 2913903 B1 (Patent Literature 1), a laser is used as a light source, and an interference filter that allows only laser light to pass therethrough is disposed in front of a lens of an imaging device. By this, optical components other than section light are cut off, eliminating disturbance components.
In JP 7-324915 A (Patent Literature 2), laser slit light is used as a light source, and an optical filter is combined with a lens of an imaging device. By this, disturbance components and noise components are removed.
In JP 4896828 B1 (Patent Literature 3), in order to remove disturbance noise that cannot be removed by an interference filter or an optical filter, a small region that limits a light-section line search range is set, and an average value or centroid value of a luminance distribution in the small region is recognized as the position of a light-section line. Furthermore, a variance value of the luminance distribution is determined, and a next adjacent small region is determined based on the variance value. Note that a point with a maximum luminance is searched from a luminance distribution in the entire screen, and a region including the point is set as a measurement start position of a light-section line.